Image of the Month - November 2004

Triple Stack

Minor planet 2004 TD18 had been listed with potential impact solutions on the JPL risk page
from 15th October 2004, 2 days after discovery with the
Spacewatch 0.9m telescope at Kitt Peak. The Spaceguard
Central Node had posted an observing campaign for 2004 TD18 on October 18, asking for more astrometry to help refine it's
orbit before the object faded from view.
Listed as magnitude +21.1 by the Minor planet Center, Astrometrica's track & stack feature enabled Peter Birtwhistle to recorded the
NEO in the night of October 21/22 with a S/N ratio of 9.0 in this stack of 120 exposures of 30 seconds each. Birtwhistle used a CCD
camera on the 0.3m f/6.3 Schmidt-Cassegrain telescope of his garden observatory located
in Great Shefford, England. With the astrometry from these images, and
another set from the Spacewatch II telescope on October 23, JPL was able to remove the last predicted impact solutions.

On the left panel in the picture above, the images have been stacked to compensate the motion of the NEO, moving 2.1" per minute in PA 230&deg.
Most of the trails in the image were caused by field stars, but to the lower right of 2004 TD18 there is a trail in a somewhat
different direction: It was caused by another minor planet, (42454) 4134 T-3. The trail of that 16mag main belter easily shows up in the
middle panel, where the images have been stacked without compensating for any motion - but the faint NEO, of course, is lost now. In the
right panels, the frames were stacked to compensate for the motion of (42454) 4134 T-3 (0.5"/minute in PA 259&deg), which is now visible
as a bright dot among shorter star trails. "The two minor planets passed each other by about 18" about 1/2 hour after the last image was
taken, at ~01:14 UT on 2004 October 22", notes Birtwhistle. The field shown in these frames covers 10' x 10', and id enlarged by 50% from
the original image scale.